1. Field of the Invention
This invention relates to an automatic control device operable on the clearance of a labyrinth type of a turbo machine seal.
2. Description of the Prior Art
Seal means between fixed and rotary parts of turbo machines frequently take the form of labyrinth seals comprising on the rotary part annular tip members in numbers varying according to operational conditions and in accordance with various operational technologies, and on the fixed part of the machine disposed opposite to the tip members, a member serving as a wear and fluid-tight seal, generally known as an "abradable". Such abradables are wearable by friction in the event of contact with a tip member (herein referred to as a tip) without giving rise to appreciable damage to the latter. Herein, the annular wear and fluid-tight seal will be referred to as "the wear seal member". Such labyrinth seals can be disposed for example between various movable stages of a compressor or a turbine, and fixed parts (or parts rotating at a different speed) adjacent thereto. The tips are in this case carried by intermediate rings or other means and the wear seal member is secured on the stator (or the rotary part moving preferably at the lower speed).
In a particular utilization of such labyrinth seals which forms an application more directly envisaged by the invention, these seals are disposed between various enclosures of the turbo machine and are to be found in particular at the ends of the outer enclosed spaces of the combustion chamber. On the one hand, on the outlet side of the compressor and, on the other hand, on the inlet side of the turbine. In this case the actual fluid-tight function of the labyrinth seal is more complex. In practice, balancing of the pressures between the various enclosures of the turbo machine is conventionally sought. A controlled air flow is also sought within the enclosures with a view to generating necessary cooling air flows eventually used in other zones of the turbo-machine and thus it may be desirable to control with high precision the air flows termed "loss flows+ traversing a labyrinth seal and of which the precise control affects various parameters such as the efficiencies of the turbo machine or the useful life of the various parts. Apart from these various operational conditions, such as the pressure within the enclosures, one of the fundamental parameters from which this control of the air flows depends is the clearance during operation between the upper part of the tips and the wear seal member.
Various proposals have been made with a view to overcoming these problems and in particular maintaining a control value of the clearance between the tips and the wear seal member in a labyrinth seal, whatever the operational conditions of the turbo-machine, at a stabilized rating or during transitory phase ratings. Thus, FR-A-No. 2 437 544 invented by the present Applicant describes a labyrinth seal in which the carrier of the wear seal member is surrounded by an annular duct connected, at its downstream end, to an air supply provided in the wall of the combustion chamber casing while its other end discharges upstream of the labyrinth seal into the space with air at a lower pressure surrounding the shaft of the compressor. The control of the amount of cooling air flow at the labyrinth seal relies in this case upon a controllable discharge valve operably dependent upon an operational parameter of the turbo machine. This control method has however, various disadvantages inherent in the method because it relies, on the one hand, upon a complex control chain thus multiplying the risks of failure or defective operation of the valves and other accessories and, on the other hand, the response time, particularly during transitory phase ratings, may be too long to ensure fully satisfactory operation.
Another device proposed in FR-A-No. 2 025 869 seeks to minimize the difference in thermal expansions in a labyrinth seal by an equalization of the temperatures between a casing supporting the wear seal member and a ring carrying the tips and connected to the rotor. With this objective, the outer surface of the casing is isolated from the flow of hot gases by a screen defining a space in which cooling air circulates. This proposal, however, does not provide any specific adaptation as a function of variations in the operational conditions of the turbo machine, in particular during transitory phase ratings.